LONDON — When Theresa May updated the House of Commons on Wednesday on Britain’s response to the horrific events in Brussels, there was little mention of the European Union referendum. A question that invited the British home secretary to beat the drum for British membership was politely brushed aside.

It was, most people seemed to agree, too soon to play politics with such a disaster — and those who attempted to do so in the immediate aftermath found themselves roundly condemned.

But as so often in politics, what no one was saying was what everyone was thinking. For political professionals, moments of horror and compassion are inevitably followed — even accompanied — by a rapid calculation of the likely consequences. Certainly the markets knew what they thought: sterling fell and bookmakers cut their odds as a consensus took hold that the Brussels attacks made Brexit more likely.

In this case, the conventional wisdom is probably right. That’s because the one-word case for remaining in the European Union, as endorsed by focus groups, has been “security.” The message has been repeated and repeated that leaving would be an unacceptable risk to the economy, and to Britain’s safety. That’s why 13 senior generals were prevailed upon to write to the Telegraph warning of Brexit’s consequences; why the defense secretary, Michael Fallon, insisted that membership helps Britain tackle global threats; and why David Cameron warned of floods of asylum-seekers arriving if the border moves back from Calais to Dover.

And this is precisely why a renewed refugee crisis, or a terrorist attack, are the two things that the Remain camp most feared: because they turn Europe from a source of security into a source of threat.

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Already, Leave campaigners such as Michael Howard, a former Tory leader, were attempting to defuse the security argument by claiming that the EU “is failing to keep its people safe” — in a speech written before the Brussels attacks but delivered afterwards, he quoted a former head of Interpol’s claim that the Schengen free-movement agreement “is like hanging a sign welcoming terrorists to Europe.”

It is an increasingly popular theme: George Eustice, a Euroskeptic minister, responded to the Brussels attacks by claiming that Schengen enables terrorists “to move around freely.” Even the Europhile Labour Party responded to May’s statement in the Commons by urging that Britain’s borders with Belgium be tightened.

The Remain camp could try to use the Brussels attacks — once a respectful period has passed — to explain why Britain’s security is enhanced by being in the EU. They could point out that the U.K. isn’t actually in Schengen; that it benefits from international anti-terrorist cooperation; and there are huge distractions, complications and uncertainties involved in unpicking the existing deals.

This, indeed, was already the strategy. In an interview with the Times Wednesday, carried out before the attacks, May defended Britain’s EU membership by pointing out that things like information-sharing and the European arrest warrant (long a right-wing bugbear) enhance Britain’s ability to identify and remove extremists. She highlighted in particular the Prüm convention for sharing fingerprints and DNA, which comes into force next year, which will cut the time for a DNA match to be returned from 143 days to 15 minutes. Meanwhile, Britain’s most senior police officer, Bernard Hogan-Howe, of London’s Metropolitan Police, said renegotiating such arrangements would be a “bureaucratic nightmare,” with no guarantee of retaining the appropriate access.

The problem is that these arguments are of the head, not the heart. It’s much easier to look across the Channel and fret about bombings in Brussels and attacks in Paris, and the prospect of more Islamist fanatics arriving from Syria via Turkey. After the Paris attacks in November, there were similar fine words about solidarity and compassion – but support for Brexit still spiked up.

By the time of the vote in June, the Brussels attacks may have receded from memory — but the undercurrent of anxiety over migration and Islamism is likely to remain. If nothing else, it provides an easy way to galvanize conviction among Euroskeptics, and drive up turnout.

Robert Oxley, spokesman for the Vote Leave campaign, told POLITICO that while it believes the European Union is making Britain less safe, it would be inappropriate to drag the tragedy in Brussels into the debate.

But in his own statement, Arron Banks, head of rival pro-Brexit group Leave.EU, said that while “the political elite and media sneer that this is not the time for making political points … this threat is a consequence of their political indecision.”

The government itself, Banks claimed, admits that free movement is “extensively exploited” by criminals — “and what goes for organized crime goes for organized terror, too. There’s virtually nothing we can do to stop every Islamist sympathizer in Molenbeek from setting up shop in the U.K. if they so choose, provided they flash an EU passport on their way in.”

And with the opening of borders to Turkey and Ukraine, and the naturalization of Syrian refugees, “the danger to the British public will only grow over time.” He ended by quoting the US politician Bobby Jindal’s argument that “immigration without assimilation is invasion.”

The head can point to Britain’s own problems with home-grown extremism — or the fact that Brexit is highly unlikely to lead to Britain actually closing its borders to Muslims, Donald Trump style, or anything like it. But in a chaotic, turbulent world, even the idea of shutting ourselves away can seem terribly enticing.

Robert Colvile is a regular contributor at POLITICO.

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Filippo

I just read an article providing evidence that in case of brexit president Macri will occupy the Falklands. It was very frightening, I think that if I was british I would have changed my mind and vote remain

Posted on 3/23/16 | 5:53 PM CET

heaven to hell

Well, sometimes, I need to question myself if the media economy is not complicit in those terror attacks. I mean bringing to near collapse an entire air-port building and a section of the underground in the very centre of Brussels is a horrific deed, but what about keeping the information at a low level and declaring holidays preemptively until the last suspect is being captured? At the moment there seem to be more of those serial killers on the loose rather than in police custody.

So we have to complicit parties here: the media economy and those networks backing or hiding these coward murderers.

Posted on 3/23/16 | 6:10 PM CET

Marcel

The only ones who want Britain to stay in are elitists like Miliband (dreaming of an unelected tax-exempt EU job), American banksters (dreaming of squeezing bailouts out of Brussels), journalists (afraid of not getting invited and feted at EU summits) and other assorted elements hostile to democracy.

They’ll line ’em all up with mission ‘Scaremonger Brits to the max’: generals who dream of a Merkelreich army commission, economists who want to be appointed to EU advisory bodies, corporate types hoping to squeeze more subsidies out of EU regions (that Ryanair criminal for example) etc.

Plus the EU is shoveling money to Amnesty, Greenpeace ea with the attached but unspoken requirement to campaign against democracy.

Posted on 3/23/16 | 6:26 PM CET

Rist

Only a deluded, obsessed mind can relate anything that happens in the world to the EU.
The EU is one among the many international organisations that there are around, and not even the most powerful.
However, I understand that minions need an easy target as a safety device, so that they steam a bit in the evining and then wake up the next morning and keep working for a few pound for their powerful (Europhobic by necessity) landlords.

Posted on 3/23/16 | 8:29 PM CET

Roy Jacobs

Pointing out that Schengen Zone allows for free movement of Kalashnikov rifles and undesirable dangerous people is not below the belt.
Pointing out that the Euro has made a small number of people very rich and millions very poor isn’t either.
Pointing put that the Euro and the Schengen Zone are the two achievements most glorified by the Europhile maniacs is fair game I would have thought.

Posted on 3/23/16 | 10:30 PM CET

Jerry Jay Carroll

The establishments will always find a way to defend the status quo if for no other reason than sluggish bureaucracies are difficult to turn a new direction.

Posted on 3/24/16 | 1:41 PM CET

My Opinion

The EU is a failure, hopefully the UK will vote to leave.
EU ideology is dangerous, EU expansionism is dangerous, they have allowed themselves to be pressured by Turkey to help them join faster in order to solve Merkels stupid “welcome all” message. Do we really want the EU to have its future borders right on the middle east and Russia ? Vote Out of the EU

Posted on 3/24/16 | 2:48 PM CET

DBKing7

Some background on the Bobby Jindal reference, and its relevance. Bobby Jindal was born in the USA to Indian immigrants and went on to be Governor of the state of Louisiana. He therefore has a direct perspective on the importance of the quote “immigration without assimilation is invasion”.

Just welcoming all like Merkel and many in the US promote – without a plan – is disingenuous to both the immigrants and the current citizens – because in the end it is doomed to fail. Slow and steady wins the race as they say.

Quick Brexit comment. I am not a Brit, but my suggestion is you consider where the EU will be 5 or 10 years from now when considering how to vote. By then Merkel (defacto EU PM) will be long gone, meaning there will be a giant leadership vacuum. Political challenges in the future will IMO pull the EU apart, as the positives of the Union will be overtaken by the negatives.